As an index for evaluating the performance of a light source, a radiation characteristic is known. The luminous intensity distributions are a typical example of such a radiation characteristic. The luminous intensity distributions mean variation or distributions of the luminous intensity depending on the angle. As such luminous intensity distributions, both the absolute luminous intensity and the relative luminous intensity are used. The luminous intensity distributions of the absolute luminous intensity are used for determining the total luminous flux generated by a light source, for example. In contrast, the luminous intensity distributions of the relative luminous intensity are used for determining a light distribution pattern, for example.
The related art of the apparatus for measuring such luminous intensity distributions are disclosed for example in Japanese Patent Laying-Open No. 07-294328 (PTD 1) and Japanese Patent Laying-Open No. 2003-247888 (PTD 2).
The Japanese Industrial Standards specify measurement of the luminous intensity distributions under JIS C8105-5: 2011 “Luminaires-Part 5: Gonio-photometric methods” (NPD 1).
Generally, in the case where the luminous intensity distributions of a light source like a large-sized lighting device is to be measured, a measurement apparatus is employed that is configured to have a plane mirror rotated to thereby direct the light emitted from the light source to an optical receiver. In such a measurement apparatus, typically the light source to be measured is arranged rotatably about a vertical axis while the plane mirror is arranged rotatably about a horizontal axis. The light distributions measurement apparatus is roughly classified into two types. One is a system in which a plane mirror is rotated at the center and a light source is rotated around the plane mirror (hereinafter also referred to as “Moving Sample System”), and the other is a system in which a plane mirror is rotated around a light source (hereinafter also referred to as “Moving Mirror System”).
More specifically, the Moving Sample System is configured so that a line connecting the center of the plane mirror and the optical receiver is identical to the rotational axis of the plane mirror. It is easy for this system to change the photometric distance (the distance from the light source to the optical receiver) depending on the size of the light source or the amount of light from the light source. In the case of the Moving Mirror System, a line connecting the photometric center of the light source and the optical receiver is identical to the rotational axis of the plane mirror. This system cannot change the photometric distance.